The present invention relates to non-contact methods and apparatus for nondestructively investigating the internal properties and defect structures of materials. The technique typically uses a pulsed laser to generate a pressure pulse at the surface of the material being interrogated and a non-contact receiver such as an electromagnetic pickup coil or other transducer to detect the scattered or reflected signal. The method is especially useful for nondestructively evaluating the properties of materials that are at elevated temperatures.
The amplitude of the pressure pulse that is generated at the material surface is maintained below the material's elastic limit in order to avoid permanent changes in the material's properties yet is of sufficient amplitude for the scattered or reflected signal to be detected by a low efficiency non-contact device.
The pressure pulse probes the internal structure of materials in a manner similar to conventional ultrasonic non-destructive testing. The intensity of the laster induced source of ultrasound, however, can be made at least a factor of 10 more intense than standard sources of ultrasound. This permits a low efficiency non-contact detector such as an electromagnetic coil to be used to detect the scattered or reflected signal without stringent requirements on signal amplification or elaborate processing schemes to distinguish the signal from background noise. Furthermore, a pulsed laser generates a short discrete pulse of ultrasound. This allows precise time discrimination schemes to be used in processing of signals, particularly in pulse-echo techniques.
Past techniques for generation of ultrasound have included dielectric breakdown in liquids, generation of thermo-elastic waves at unconfined surfaces wherein a surface layer of material is rapidly and nonuniformly heated to temperatures below its vaporization point, and vaporization of a small amount of surface material by the impinging laser beam. Transparent materials placed directly on the surface of the absorbent material also have provided an effective method of enhancing the amplitude of the signal that is generated at an unconfined surface.
The present method of generating an ultrasonic signal does not rely on the use of a transparent overlay (which in many cases of nondestructive evaluation, e.g., high temperature materials, would not be practical) to generate an intense source of ultrasound. Also the present technique does not disturb the surface of the material being evaluated, which occurs when the surface is vaporized by the incident laser beam. The present method involves initiation of a blast wave in the atmospheric environment in the vicinity of the surface of the material being evaluated. Pulsed carbon dioxide and neodymium-glass lasers are effective systems for generating intense ultrasonic signals by this method. Other pulsed lasers with wavelengths intermediate to these two systems such as carbon monoxide lasers also provide attractive systems for generating intense pulses of ultrasonic energy in materials. A significant fraction of such signals generated by systems of these types contain low frequency components which can propagate through large thicknesses of material. This is an important consideration when large parts are being evaluated.
A major problem encountered in the steel industry is the inability to nondestructively interrogate hot steel billets to detect internal defect structures before cooling. The present technique can detect defect structures in hot steel.